CH bond lengths and strengths, unperturbed CH stretching frequencies, from partial deuteration infrared studies: t-Butyl compounds and propane

Abstract

Abstract Infrared spectra have been recorded of the CH stretching bands of CHD2 groups found in (CD3)3 CX compounds, where X  OH, F, Cl, Br, I, D, and of the propane derivatives (CD3)2CHD and CD3CD2CHD2. Each methyl group contains two different CH bonds, whose lengths r° and dissociation energies D°298 are predicted, as are those of the 2 CH2 and 3 CH groups. The various bonds are identified firstly by use of the band contours in the gas phase: secondly by simple normal coordinate treatments (“3 × 3 refinement”) of the CH and CD stretching motions; and thirdly, in propane, by use of crystal spectra. The unperturbed positions of CH3 and CD3 stretching vibrations affected by Fermi resonance are predicted. A study of the variations in bond strength reveals the systematic weakening of alkane CH bonds by α methyl substitution, the trans effect of a CX bond, whose magnitude depends critically on the X substituent, a gauche effect of the latter which depends little on the halogen involved, a stronger CH bond trans to CH3 than one trans to H in the same molecule and a regular displacement of gauche and trans CH bond frequencies with progressive methyl substitution. The earlier CHD2 data on methylamines are reconsidered and reinterpreted.